Stuck egg?

You can treat her with warm water baths.  She needs to sit in there for 20 minutes and them get dried off.  Its best to keep her away from the rest of the flock during this and you may need to repeat it several times.  While you are drying her off, you can massage round the area between her legs and you may feel a lump if it is an egg.

The hen needs to go into a cardboard box or similar after her bath to see if she can then pass the egg.

If it persists, I would think about a vet visit.  If the egg breaks inside her, she will need antibiotics to fend off infection.

Fingers crossed she perks up soon